1. Field of the Invention
This invention relates to a valve actuating apparatus for an internal combustion engine, which controls opening/closing operations of an intake valve, particularly valve-closing timing of the same.
2. Description of the Prior Art
Conventionally, a valve actuating apparatus of the above-mentioned kind is disclosed e.g. in Japanese Utility Model Publication (Kokoku) No. 56-9045. This valve actuating apparatus comprises a camshaft that rotates in synchronism with rotation of a crankshaft of an internal combustion engine, a pair of cams having different cam profiles, a pivotally movable rocker arm that comes into contact with a selected one of the two cams and is held in contact with an intake valve, a rocker shaft that pivotally movably supports the rocker arm, and a switching mechanism that switches a cam for contact with the rocker arm, between the pair of cams.
The pair of cams are comprised of a normal cam and a delayed-closing cam. The normal cam has a cam profile designed for a maximum power output of the engine in medium-load and high-load operating regions, while the delayed-closing cam has a cam profile designed for delayed valve-closing timing of the intake valve compared with valve-closing timing provided by the normal cam, for operation of the engine in the Miller cycle operation. These cams are integrally formed on the camshaft, for rotation along with the camshaft in synchronism with rotation of the crankshaft. According to the above configuration, the intake valve is opened and closed by a selected one of the cams via the rocker arm in timing corresponding to the profile of the selected cam.
The switching of the cam for contact with the rocker arm between the two cams is performed by the switching mechanism according to the load on the engine. This switching mechanism is comprised of a bracket provided on the rocker shaft, a control ring coaxially provided on the rocker shaft on a side of the rocker arm remote from the bracket, springs disposed, respectively, between the bracket and the rocker arm and between the control ring and the rocker arm, and an actuator connected to the control ring. The actuator actuates the control ring along the axis of the rocker shaft to move the rocker arm along the axis of the rocker shaft against the urging forces of the springs, whereby the cam for contact with the rocker arm is switched between the two cams.
Due to this configuration, in the medium-load and high-load operating regions of the engine, the intake valve is opened and closed by the normal cam for operation of the engine in the normal Otto cycle, while in a low-load operating region, the same is opened and closed by the delayed-closing cam. When the intake valve is opened and closed by the delayed-closing cam, the engine operates in the so-called Miller cycle in which valve-closing timing is delayed, whereby most of a mixture drawn into a cylinder is returned into the intake pipe by the compression stroke of a piston, whereby optimal intake air compression in the low-load operating region can be achieved. This makes it possible to reduce pumping loss, which occurs when the normal cam is used, to thereby improve fuel economy.
However, the above conventional valve actuating apparatus suffers from the following problems: When the engine is in the low-load operating region, fuel economy can be improved during cruising of the vehicle, as described above, but during the starting and idling of the engine, opening and closing of the intake valve by the delayed-closing cam causes degradation of combustion in the cylinder, resulting in unstable combustion. Further, when a delayed-closing cam modified in cam profile for reduced delay in valve-closing timing is used so as to prevent degradation of combustion, pumping loss during cruising or in the low-load operating region of the engine cannot be sufficiently reduced, which reduce the above-mentioned effect of the improvement of fuel economy. In short, during cruising of the vehicle, or in the low-load operating region of the engine, it is difficult to achieve both of stable combustion and improved fuel economy at the same time.